Maintaining Roadway Infrastructure and Parkway Trees
Public Works strives to maintain healthy parkway trees along with providing safe travel for motorists and cyclists on County roadways and safe access for pedestrians on County sidewalks. To accomplish this, the street pavement, concrete curb, gutter, and sidewalk are routinely inspected, maintained, and periodically replaced. As parkway trees in our urban forest age, their canopy and roots grow. Often the parkway tree roots grow beyond the narrow green space between the sidewalk and the curb and in the process, uplift the adjacent infrastructure.
Why are parkway trees root pruned?
Parkway trees are sometimes root pruned in order to accommodate an adjacent infrastructure repair. Before a tree is root pruned, it is evaluated by the Urban Forestry Unit to determine whether the tree can be safely root pruned per ISA standards to allow the replacement of the adjacent sidewalk and/or curb and gutter. If the tree cannot be safely root pruned, it may be removed and in most cases, replaced.
Why can't all parkway trees be safely root pruned?
Tree roots may grow two to three times beyond the drip line of the tree canopy and most exist in the top 18” of soil. There are many factors that affect a tree’s tolerance for root pruning including the following:
- Root size – The larger the tree roots cut, the greater the stress on the tree.
- Number of roots cut – The more roots pruned, the greater the stress on the tree.
- Proximity of the cuts to the tree trunk – The closer the cuts to the trunk of the tree, the greater the stress on the tree.
- Species of the tree - Certain species can tolerate root pruning better than others.
- Age of the tree – The older the tree, the less tolerant the tree is to root pruning.
- Condition of the tree – Trees that are in poor or declining health should not be root pruned.
- Lean of the tree – Leaning trees should not be root pruned.
The appropriate Road Maintenance District Urban Forestry Unit will evaluate all of these factors in determining whether a tree can be safely root pruned.
Sustainable community solutions
Because the County has approximately 170,000 trees, over 2,600 miles of sidewalk, nearly 3,400 miles of curb and gutter, and over 9,500 lane miles of roadway to maintain, our choices for how to maintain the trees, the street and parkway need to be sustainable.
Parkway tree species selection
When parkway trees are replaced, Public Works makes every effort to select tree species that are both complimentary to the other parkway trees in the neighborhood and appropriately sized at maturity for the parkway. A list of recommended parkway tree species along with tree photos can be found on our website by using the Tree Services Locator
Parkway trees are not the only trees that sometimes damage public infrastructure. At times, roots from private trees extend under and displace sidewalks. When this occurs, Public Works will notify the underlying property owner and provide them with the opportunity to root prune their tree so that Public Works can replace the sidewalk without damaging the private tree. If the tree roots are not pruned, Public Works may opt to replace the concrete sidewalk with asphalt in order to better preserve an accessible sidewalk surface.